Geophysicists have held since the 1940s that Earth’s interior core is a partly crystallized sphere of iron and nickel that is gradually cooling and expanding. As it cools, this inner core releases energy to an outer core called the fluid core, which is composed of iron, nickel, and lighter elements, including sulfur and oxygen. Another model called the “nuclear earth model” holds that there is a small core, perhaps 5 miles (8 kilometers) wide, of uranium and plutonium surrounded by a nickel-silicon compound. The uranium and plutonium work as a natural nuclear reactor, generating radiating energy in the form of heat, which in turn drives charged particles to create Earth’s magnetic field. The traditional model of Earth’s core is still dominant; however, scientists have yet to disprove the nuclear earth model.